This invention relates generally to antenna structures for wireless communications devices, and more particularly to compact, high efficiency, electrically small loop antennas for use in conjunction with portable wireless communication devices.
The physical size of modem compact communication devices often is dictated by the size of the antenna needed to make them function effectively. To avoid devices that are too large, pagers and other devices have made use of electrically small rectangular loop ({fraction (1/10)} wavelength). However, these small antennae tend to be inefficient as a result of their very low radiation resistance and comparatively high resistive loss. Likewise, as a result of their high Q they tend to be sensitive to their physical environment.
Yet another consideration and challenge of designing modem wireless communications devices is the efficiency of packaging the necessary components within an increasing smaller physical package. As the overall size of wireless devices has continued to decrease, a particularly difficult challenge to those skilled in the relevant arts has been the efficient placement of components, such as batteries, antenna structures, RF signal reception and transmission circuits, and other digital and/or analog devices or module, within the overall device package. Especially important has been the placement of antenna structures and assemblies relative to the RF signal generating components or modules. Those skilled in the relevant arts recognize the difficulties in preventing electromagnetic coupling between these components.
To overcome the disadvantages of electrically small loop antennas, there is a continuing need for antennas small in physical dimension; having relatively high efficiency; capable of being placed in close proximity to associated electronic circuits without adversely effecting performance; easy to manufacture using standard, low-cost components; and capable of having radiation patterns altered to support different applications.
For some wireless communications devices, an antenna assembly may be located remotely from the electronic device or devices it is serving. Remote location of the antenna assembly relative the associated wireless device may minimize coupling of RF energy into digital or other circuitry from the strong fields around the antenna or for the antenna to have access to signals. GPS receivers, and BLUETOOTH(copyright) or other types of UHF and microwave digital transceivers, particularly those having the antenna integrated on or within the device, often benefit by having the antenna assembly remotely disposed relative to the RF signal generating/receiving components.
One limitation associated with a remote antenna is that power is lost through the transmission line connecting the antenna assembly to the electronics. Obviously, this is undesirable as it degrades the performance of the system by increasing the noise and reducing the transmit power. A decrease in important antenna parameters, such as gain, results from power loss in the transmission line. Signal reception is also negatively impacted by transmission line losses.
Also known are circular polarization antenna structures or systems for reception of left hand and right hand polarized signals. Circular polarization is typical of satellite systems, such as the Global Positioning System (GPS). This field is in rapid expansion due to the vast range of possible applications and the relative low cost of implementing these systems.
The fixed and mobile land devices associated with such systems have required more specialized antennas designed to perform specific functions effectively. Two types of antennas have to date been used in circular polarization communication and navigation systems on mobile devices: the first is the xe2x80x9chelixxe2x80x9d or helicoidal antenna, while the second is known as the xe2x80x9cpatchxe2x80x9d antenna.
In helicoidal antennas, circular polarization is obtained by exciting a progressive wave on a helicoidal wire; the direction of the circular polarization (left or right) is determined by the sense of helicoidal wire winding.
The helicoidal antennas have the advantage of being very simple to design and produce and have a considerable band width which ensures high sensitivity; this characteristic of the helicoidal antenna makes the tolerance range wider, making it possible to use inexpensive materials which are easy to obtain on the market. This type of antenna has the added advantage of having a good gain value in an axial direction with an equally good axial ratio that, as the experts in the field know, is the most important reference parameter for the quality of circular polarization.
The disadvantage of helicoidal antennas is their by no means negligible height which makes them inconvenient for certain applications, such as installation on vehicles or hand-held devices where low profile antennas are required, obviously because they must be streamlined.
The low profile is the main characteristic of the second type of antenna mentioned above, known as the patch antenna, where circular polarization is obtained by exciting a resonant current distribution on a planar conducting surface. The direction of circular polarization is determined by a precise calculation of the position of the xe2x80x9cpoint of excitationxe2x80x9d of the surface.
This type of antenna, however, requires the use of relatively expensive materials, and, above all great precision during setting up and production due to the small tolerances to respect.
Considering the above state-of-the-art, another type of circular polarization two-way antenna was designed with the aim of offering all the advantages of both of the above antennas, without the disadvantages or application limitations of either.
An omni-directional antenna which includes a conductive loop element supported above a conductive ground plane of a wireless communication device by a conductive leg member. The conductive leg member further defines a feedpoint at which the antenna is operatively coupled to the device""s signal generating circuitry. A dielectric element may optionally be disposed between the loop and ground plane.
The improved antenna displays gain in both the vertical and horizontal orientations. The horizontal gain is due to currents in the loop. The vertical gain (perpendicular to the loop and the ground plane) is due to displacement current fields within the conductive leg member disposed between the loop and the ground plane.
Circular polarization is obtained by exciting a wave along a loop wire. The loop defines a closed path, which need not necessarily be a circular path. An antenna including a rectangularly defined loop is also disclosed herein. Different approaches may be utilized to effect wave polarization (left-hand or right-hand); the first consists in exciting the loop wire at two separate points staggered at an angle of 90 degrees with respect to the center of the loop wire and providing a source in phase quadrature. Alternatively, the loop wire may be excited at only one point by discriminating one of the two polarizations by means of a passive probe, a directional probe or other suitable means.
The operational frequency band of the antenna is largely determined by the outside circumference dimension of the conductive loop. The outer circumference dimension is substantially equivalent to xc2xd of the wavelength of the frequency of response. Thus the system performs similarly to a xc2xd wave slot antenna. Tuning of the antenna can be accomplished by adjusting the feed network. Adjusting the width and location of the conductive leg member will transition the frequency and impedance.
Another aspect of the present invention addresses the problem of power loss in the transmission line connecting the antenna to the RF electronics. In embodiments of the present invention, this concern is addressed by proving a low noise amplifier (LNA) proximate the antenna structure. In one preferred embodiment, the LNA may be disposed within the antenna structure, such as within a cavity defined in a portion of the antenna.
Another aspect of the present invention is the provision of an antenna wherein various components of the transceiver and/or handset, such as a LNA, are placed within or under the antenna, importantly without negatively impacting antenna performance. A LNA with or without pre/post filter may be disposed within a cavity of a portion of the antenna assembly. In one preferred embodiment, the electronic or other components, may be disposed within a cavity defined within a disk-shaped dielectric element.
Yet another aspect of the present invention is the provision of an antenna structure having a conductive loop resonator element disposed in operative relationship to a conductive ground plane element. In one embodiment, the conductive ground plane may be the ground plane of a wireless communications device. In another embodiment, the conductive ground plane may be a separate conductive panel or element which is coupled to the ground plane of the wireless device. For example, the antenna may be remotely disposed relative to the wireless device and coupled thereto by a transmission line, such as a coax signal line, etc.
Yet another feature of embodiments of the present invention is a notch element in the conductive leg member. Changes in the notch height can be used to adjust the antenna match. Further tuning can be accomplished by adjusting the width of the ring. As the ring is made wider, the operational frequency range becomes higher.
One advantage of the invention is that the antenna performance is largely independent of the dimensions of the ground plane. Thus the antenna can be readily adapted to different devices having various ground plane dimensions.
The above and other objects and advantageous features of the present invention will be made apparent from the following description with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.